The invention relates to a satellite-based navigation method for the determination of the position of a receiver by ascertaining the signal propagation time between preferably at least two satellites and the receiver.
As a rule, in satellite-based navigation, the signal propagation time between several satellites and the receiver is determined, and from that determination the position of the receiver is calculated. For each satellite i from the relation c·(Treceiver−Ttransmitter) the pseudo-distance is determinedρi=√{square root over (( xi− x)2)}+c·Δt+εwhere xi is the position of satellite i, x the position of the receiver, c the speed of light, Δt the time error of the receiver clock, and ε is other errors (such as orbit and clock errors of the satellite, propagation time errors due to atmospheric conditions or other errors of measurement of the receiver).
In general, for reasons of cost in the receiver, highly precise time references are omitted, such that the dominant error in position determination is the time error Δt in the receiver clock. This time error is included in the calculation of position. Therefore, for the position determination, at least four satellite signals are required in order to determine the three spatial coordinates and the time error and to determine therefrom Treceiver.
Through differential satellite navigation methods, errors in the satellite segment and propagation time errors due to atmospheric conditions can be detected and largely compensated. If redundant information is available (i.e., if for a three-dimensional position determination more than four satellites and for a one-dimensional, track-guided position determination more than two satellites are received) through RAIM algorithms (Receiver Autonomous Integrity Monitoring) in the receiver, an error self-detection can be carried out, which uncovers propagation time errors of the received satellites. If, however, the signal propagation times to several satellites are falsified, these errors cannot always be detected with certainty such that, in spite of the RAIM algorithm, unrecognized false position determinations result.
U.S. Pat. No. 5,736,960 A discloses a satellite-based navigation method for the determination of the position of a receiver by ascertaining the signal propagation time between satellites and the receiver, the receiver having a precise time reference and an absolute time signal from a fixed reference station with precisely known position being utilized.